M2X (M= Li, Na; X= F, Cl): the smallest superalkali clusters with significant NLO responses and electride characteristics

Abstract

Hypervalent M2X (M = Li, Na; X = F, Cl) clusters are prototype species possessing lower ionisation potentials than Li, therefore classified as superalkalis. This study reveals some interesting properties of these small clusters using ab initio MP2/aug-cc-pVTZ and QCISD/aug-cc-pVTZ methods. These clusters are shown as an ionic species, composed of positively charged cage of alkali metals (M2+) and halogen anion (X−). Therefore, the stability of M2X is governed by both ionic and covalent interactions. We show that the excess valence electron of (M2+) is pushed out by anionic X−, which allows M2X clusters to possess ‘electride’ characteristics. It is also due to this excess electron that M2X clusters exhibit significant non-linear optical (NLO) properties. The dipole moment, mean polarisability and hyperpolarisability suggest their significant NLO responses, which are explained on the basis of electronic transitions in crucial excited states using TD-B3LYP/aug-cc-pVTZ method. The first static hyperpolarisabilities of Li2F and Na2F take the values of order of 104 a.u. due to their lower transition energies. This study should provide new insights into the design of novel materials with significant NLO responses useful for electro-optical applications.